Anyone got a PHD in .... PHD

[Astroarg]

OK, so I get the grasp of PHD2 and its designed to work out the box, but I have a few questions around the math.

 

I have an Altair 72EDF scope (or camera lenses up to 200mm) and I use the Altair MG32 guide package.

 

My math so far has come to:

 

Guidescope

Guide scope focal length 126mm.

Guide Cam pixel size = 3.75

Pixel scale 6.14" /px

(3.75 / 126) * 206.3 - 6.1398

 

Scope + Imaging Cam

2.4 image camera pixel size (183C)

Focal Length 345.6 (0.8x reducer)

Focal Length 420 (1x flattener)

 

Pixel scale:

(2.4 / 345.6) * 206.3 = 1.4326

(2.4 / 420) * 206.3 = 1.1789

 

So my imaging resolution isn't great?  I assume I'm pushing on the door of the "rule of thumb imaging pixel scale should be no more than 4x your guiding pixel scale"

With 0.8x reducer: 6.14 / 1.43 = 4.29

With no reducer 6.14 / 1.18 = 5.2

 

 

But AA sell the MG32 kit as "guider for up to 600mm focal length" Altair MG32 Mini Guide Polar Alignment Scope QRB Rings GPCAM Guide Camera (altairastro.com)

 

Ultimately, what would be the issues I face? I think multi-star has been helping a lot recently, I generally get a crazy 10 mins on my CEM25P (set to RA PEC) but once settled it, other than the Dithers it sits at under 0.5" all night

 

Edited March 7, 2021 by Astroarg

[AstronomyUkraine]

50 minutes ago, Astroarg said:

Ultimately, what would be the issues I face? I think multi-star has been helping a lot recently, I generally get a crazy 10 mins on my CEM25P (set to RA PEC) but once settled it, other than the Dithers it sits at under 0.5" all night

Another rule of thumb is when autoguiding, the guide scope should have a focal length of at least 1/10 of the imaging scope. When guiding was done manually, the guide scope focal length had to be at least a 1/3 of the focal length of the main scope. I can't see you having problems with your setup. I wish I had guiding issues with an RMS under0.5. 😁

[Astroarg]

Thanks.. I’m checking over a few things to still try and find source of my star shapes. I decided to look at guiding yesterday and got in a minefield with sites saying imaging resolution should be no more than 3x, then others saying 4x and one saying 5x (which if latter is correct, I’m OK)

[AstronomyUkraine]

2 hours ago, Astroarg said:

Thanks.. I’m checking over a few things to still try and find source of my star shapes. I decided to look at guiding yesterday and got in a minefield with sites saying imaging resolution should be no more than 3x, then others saying 4x and one saying 5x (which if latter is correct, I’m OK)

The way I see it, if your stars are round, does it really matter what the numbers say. Too many photographers get bogged down in stats and theory, when the human eye is the best judge.

[Astroarg]

8 hours ago, AstronomyUkraine said:

The way I see it, if your stars are round, does it really matter what the numbers say. Too many photographers get bogged down in stats and theory, when the human eye is the best judge.

 

mine aren't, I think its spacing as terrible on the edges rather than the centre, but something I read suggested it might be guiding related.

I'm heading down a path of no idea!!

[AstronomyUkraine]

8 minutes ago, Astroarg said:

mine aren't, I think its spacing as terrible on the edges rather than the centre, but something I read suggested it might be guiding related.

I'm heading down a path of no idea!!

When you calibrate PHD, have you checked the graph to see everything is OK? Also the guiding assistant is excellent for working out your optimal settings for guiding, based on your calibration data.

[Astroarg]

37 minutes ago, AstronomyUkraine said:

When you calibrate PHD, have you checked the graph to see everything is OK? Also the guiding assistant is excellent for working out your optimal settings for guiding, based on your calibration data.

yeah... think I've nailed my issue ("think") tonight as had a small window of clear skies to test.

 

I've had to go LESS spacing, not more.  I had tried 54mm, 55 (as standard) and all the way up to 60, the stars image always seemed to indicate the flattener was too close to the sensor.  Turned out it was too far away instead!!

[AstronomyUkraine]

50 minutes ago, Astroarg said:

yeah... think I've nailed my issue ("think") tonight as had a small window of clear skies to test.

 

I've had to go LESS spacing, not more.  I had tried 54mm, 55 (as standard) and all the way up to 60, the stars image always seemed to indicate the flattener was too close to the sensor.  Turned out it was too far away instead!!

Great, glad you found the culprit. Surprising just a small amount can cause so many problems.